There is increasing interest in the field of wireless charging for battery powered portable devices such as mobile telephones, tablet computers and the like. Devices capable of wireless charging need not be physically connected to a source of charging current such as a mains powered charger. Instead, such devices can simply be placed on a wireless charger, which wirelessly provides charging energy to the device, typically by inductive coupling.
In wireless charging systems, there is an ever-present possibility of incurring damage to a target device (i.e. a device being charged wirelessly) if the power being supplied by the wireless charger is too great, or is applied for too long.
To combat this risk, passive detuning or reconfiguration of the target device's antenna has been proposed. Under this proposed scheme, a passive component in an antenna filter of the target device reacts automatically to a risk condition such as excessive charging power to de-tune the antenna, thereby reducing the coupling between the charger antenna and the target device antenna, with the aim of reducing the power entering the device. For example, the target device may include a thermistor positioned close to the device battery. As the battery temperature increases the resistance of the thermistor changes, such that if the temperature of the battery becomes too high the antenna is de-tuned or re-configured automatically.
An alternative is simply to disconnect the target device's antenna from the other components in the target device if the power being supplied by the wireless charger is too great, or is applied for too long. Under this proposed scheme, one or more transistors connects the antenna to the other components in the target device (e.g. one or more transistors may connect the antenna to the antenna filter), and in the event of a risk condition such as excessive charging power as detected, for example, using a thermistor positioned in close proximity to the battery of the target device, the one or more transistors are switched off, thereby disconnecting the antenna from the antenna filter and other downstream components of the device.
One potential problem with the proposed approached is that, even when the antenna is detuned or disconnected, significant power is being supplied by the charger. If the antenna is detuned to the point that it can be regarded as being open circuit, or disconnected, a potentially damaging large voltage can develop across it, or potentially damaging high currents may develop in the antenna. In either case, in order to mitigate the risk of damage to the target device due to these high voltages or currents, high power transistors must be used to disconnect the antenna from the rest of the target device. The use of such high power transistors adds significantly to the bill of materials cost of the target device, however.
Accordingly, a need exists for some means of reducing the risk of damage to a target device due to high current or voltage levels that may develop in the target device when passive detuning or reconfiguration systems or antenna switching systems are used to protect the target device from risk conditions such as overcharging.